Magneto-optical recording medium and method for reproducing information from a magneto-optical recording medium having three layers

ABSTRACT

A magnetooptical recording medium has a first magnetic layer which is an in-plane magnetization film at both room temperature and high temperatures and changed to a perpendicular magnetization film at intermediate temperatures, and a second magnetic layer which is composed of a perpendicular magnetization film. The recording medium enables realization of high S/N reproduction of information recorded at a pitch below the diffraction limit of light with a simple structure, and further improvement in linear recording density and track density.

ThisA division of the present reissue application was filed on Jan. 15,2003 as reissue application Ser. No. 10/342,217. The present applicationis a reissue application of U.S. Pat. No. 5,831,944, which issued onNov. 3, 1988 from application Ser. No. 08/858,206, filed May 13, 1997,now abandoned, which is a continuation of application Ser. No.08/389,579 filed Feb. 15, 1995, now abandoned, which in turn is acontinuation-in-part of application Ser. No. 08/111,974 filed Aug. 26,1993, now abandoned in favor of continuation application Ser. No.08/643,833 filed May 7, 1996, which issued as U.S. Pat. No. 5,626,428 onApr. 1, 1997. A division of said application Ser. No. 08/643,833 issuedas U.S. Pat. No. 5,889,739, which is the subject of pending reissueapplication Ser. No. 09/820,734.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetooptical recording mediumallowing information recording and reproduction by irradiation with alaser beam, utilizing magneto-optic effect, and particularly to amagnetooptical reproducing method and magnetooptical recording mediumwhich enables a higher-density medium by improving linear recordingdensity and track density.

2. Related Background Art

In recent years, a magnetooptical recording medium has become a subjectof attention in the field of a rewritable recording method of highrecording density. In such a recording method, information or data isrecorded in the recording medium by forming a magnetic domain in amagnetic film of the medium by means of thermal energy of laser beamsemitted from a semiconductor laser, and information is read out from themedium, utilizing magneto-optical effect. The above-noted trend is basedon need for a larger amount of recording capacity to be achieved byhigher recording density of such a recording medium.

By the way, the linear density of an optical disc, such as amagnetooptical recording medium, is largely dependent on the wavelengthof a laser beam and the numerical aperture of an objective lens used inan optical system for reproduction information. When the wavelength λ ofa laser beam used in a reproducing optical system and the numericalaperture NA of an objective lens are determined, a bit or pitperiodicity or pitch is defined as λ/2NA which is a minimum scale orlimit of detection.

Track density of the optical disc is, on the other hand, chiefly limitedby crosstalk. The crosstalk is largely dependent on a laser beamdistribution or profile on a medium surface and is expressed as afunction of λ/2NA, similar to the bit periodicity mentioned above. Thus,the wavelength of a laser beam must be shortened and the numericalaperture NA of an objective must be enlarged in order to increase therecording density of the conventional optical disc.

However, there are limitations to improvement of the wavelength of alaser beam and the numerical aperture of an objective. Techniquestherefor have been developed, which improve the structure of a recordingmedium and a method of reading out data bits so that the linearrecording density can be improved.

For example, Japanese Patent Laid-Open No. 3-93058 discloses a methodfor improving the linear recording density and track density. Theprocess is performed in the following sequence. First, a recordingmedium, which comprises a readout layer and a recording layer, isprepared. After the direction of magnetization in the readout layer isoriented in a single direction prior to information reproduction, theninformation held in the recording layer is transferred to the readoutlayer. Thus, it becomes possible to reduce interference between codes atthe time of information reproduction and to reproduction informationrecorded at a pitch below the diffraction limit of light (magneticsuper-resolution).

However, the magnetooptical reproducing method of Japanese PatentLaid-Open No. 3-93058 requires a step of aligning the magnetizationdirection of the readout layer in a predetermined direction, which isconducted before projection of a laser beam onto the readout layer.Thus, it is necessary to add a magnet for initializing the readout layerto a conventional apparatus. Due to such addition, problems arise, suchas more complicated structure of a magnetooptical recording apparatus,difficulty of down-sizing, and higher cost of an apparatus.

The inventors previously proposed a method for realizing magneticsuper-resolution by a simple film structure without the need for anexternal magnetic field at the time of reproduction. This method uses amagnetooptical recording medium comprising two layers including areadout layer which is an in-plane magnetization film at roomtemperature and changed to a perpendicular magnetization film at raisedor high temperatures, and a recording layer which is composed of aperpendicular magnetization film. In this method, for realizing magneticsuper-resolution, the readout layer is used as an in-plane magnetizationfilm within a low-temperature area in a light spot, and as aperpendicular magnetization film within a high-temperature area fortransferring magnetization information stored in the recording layer andreproducing the information.

However, in such a super-resolution medium comprising the in-planemagnetization film, only the high-temperature area within a light spotis a reproducible area. It is thus difficult to stably provide areproduction area having a predetermined space, and signal output ispossibly decreased because the reproduction area is at the edge of thelight spot.

SUMMARY OF THE INVENTION

In consideration of the above problems, it is an object of the presentinvention to provide a magnetooptical recording medium which enablesrealization of high S/N reproduction of information recorded at a pitchbelow the diffraction limit of light with a simple structure, and amethod for reproducing information using the recording medium.

The object is attained by a magnetooptical recording medium comprising afirst magnetic layer which is an in-plane magnetization film at bothroom temperature and raised or high temperatures and changed to aperpendicular magnetization film at intermediate temperatures, and asecond magnetic layer which is composed of a perpendicular magnetizationfilm.

Further, the object is attained by a method of reproducing, by using alaser beam, information recorded on a magnetooptical recording mediumcomprising a first magnetic layer which is an in-plane magnetizationfilm at both room temperature and raised or high temperatures andchanged to a perpendicular magnetization film at intermediatetemperatures, and a second magnetic layer which is composed of aperpendicular magnetization film. The method comprises a step ofprojecting the laser beam on the recording medium from the side of thefirst magnetic layer, a step of changing both low-temperature andhigh-temperature areas within a light beam projection portion of thefirst magnetic layer to an in-plane magnetization film, and anintermediate-temperature area thereof into a perpendicular magnetizationfilm, a step of performing exchange coupling of the perpendicularmagnetization of the first magnetic layer and the magnetization of thesecond magnetic layer to transfer the information recorded on the secondmagnetic layer to the first magnetic layer, and a step of reproducingthe recorded information based on the magneto-optic effect of lightreflected from the recording medium.

The recording medium and reproducing method will be described in detailbelow with reference to embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a film structure in accordance withan embodiment of the present invention.

FIG. 2 is a schematic view showing a film structure in accordance withanother embodiment of the present invention.

FIGS. 3(a) and 3(b) are schematic views showing the wholes of filmstructures of the present invention.

FIG. 4 is a graph illustrating an example of the temperaturedependencies of 2πMs² and perpendicular magnetic anisotropy constant Kuof a readout layer.

FIG. 5 is a schematic view illustrating an example of an informationreproducing method of the present invention.

FIG. 6 is a schematic view illustrating a magnetized situation when anintermediate layer is provided between a readout layer and a recordinglayer according to the present invention.

FIGS. 7(a) and 7(b) is a schematic view illustrating a magnetizedsituation when an intermediate layer with small in-plane anisotropy isprovided.

FIGS. 8(a) and 8(b) is a schematic view illustrating a magnetizedsituation when an intermediate layer with large in-plane anisotropy isprovided.

FIG. 9 is a schematic view illustrating another example of aninformation reproducing method of the present invention.

FIGS. 10(a) through 10(c) is a graph illustrating the temperaturedependencies of Ms when an intermediate layer is provided between areadout layer and a recording layer according to the present invention.

FIG. 11 is a schematic view illustrating the relation between the beamintensity of a light spot and detection region within the light spot.

FIG. 12 is a schematic view illustrating an information reproducingmethod of a comparative example.

FIG. 13 is a graph illustrating an example of the temperature dependencyof a residual θ_(K) (when a magnetic field=0) of the recording medium ofthe present invention.

FIG. 14 is a graph illustrating an example of the temperature dependencyof a residual θ_(K) (when a magnetic field=0) of the recording medium ofthe present invention.

FIG. 15 is a graph illustrating an example of the temperature dependencyof a residual θ_(K) (when a magnetic field=0) of a recording medium of acomparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A magnetooptical recording medium and reproducing method using therecording medium of the present invention are described in detail belowwith reference to the drawings. Hereinafter, a first magnetic layer, asecond magnetic layer and a third magnetic layer are referred to as areadout layer, a recording layer and an intermediate layer,respectively.

The magnetooptical recording medium of the present invention is composedof at least two layers: a readout layer and a recording layer. Thereadout layer remains an in-plane magnetization film at roomtemperature, is changed into a perpendicular magnetization film when thetemperature is raised, and is changed into an in-plane magnetizationfilm or eliminates magnetization when the temperature is further raised.The recording layer remains a perpendicular magnetization film not onlyat room temperature but also at raised temperatures. The above-describedstates of the readout layer and the recording layer are states whenthese layers are laminated.

The readout layer is preferably composed of, for example, a rare-earthand iron group amorphous alloy, such as GdCo, GdFeCo, GdTbFeCo, GdDyFeCoor NdGdFeCo. By “iron group” is here meant the elements Fe, Co, and Ni.Material having small magnetic anisotropy and compensation temperaturebetween room temperature and Curie temperature is preferable.

The recording layer is preferably composed of material having largeperpendicular magnetic anisotropy and being capable of stablymaintaining the magnetized state, for example, a rare earth and irongroup amorphous alloy, such as TbFeCo, DyFeCo or TbDyFeCo; garnet; or aplatinum group and iron group periodical structure layer, such as Pt/Coor Pd/Co; or a platinum group and iron group alloy, such as PtCo orPdCo. By “platinum group” is here meant the elements Ru, Rh, Pd, Os, Ir,and Pt. Further, an element for improving corrosion resistance, such asCr, Al, Ti, Pt, Nd or the like, may be added to each of the readout andrecording layers as the magnetic layers.

Dielectric, such as SiNx, AlNx, TaOx, SiOx or the like, may be added tothe readout and recording layers in order to improve interferenceeffect. Material for improving thermal conductivity, such as Al, AlTa,AlTi, AlCr, Cu or the like, may be provided.

An intermediate layer for adjusting exchange coupling force ormagnetostatic coupling force, or an auxiliary layer for assistance ofrecording or reproduction may be formed. A protecting coating, which iscomposed of the above-discussed dielectric or polymer resin, may be usedas a protective film.

The following description deals with the recording-reproducing method ofthe present invention.

Referring to FIG. 1, data signals are first recorded in the recordinglayer of the magnetooptical recording medium of the present invention.One way of recording data signals in the recording layer is to modulatean external magnetic field while projecting a laser beam powerful enoughto raise the temperature of the recording layer to over Curietemperature. Another way is to modulate laser power while applying amagnetic field in the recording direction, after eliminating data in therecording layer. The other way is to modulate a laser power whileapplying an external magnetic field.

When the intensity of the laser beam is determined so that thetemperature of a predetermined region within a beam spot is raised closeto Curie temperature of the recording layer, considering the linearvelocity of the recording medium, a recording magnetic domain, which isless than the diameter of the laser beam spot, is formed. As a result,it is possible to record signals having a periodicity less than thediffraction limit of light.

When reproducing the data signal, a readout laser beam is projected ontothe recording medium. At this time, however, the temperature in theregion, irradiated with the beam, rises. Since the medium moves at aconstant speed, the temperature distribution on the recording medium hasa shape extending along the moving direction of the recording medium, asshown in FIG. 5 or 9. In the temperature distribution, a part within thebeam spot is a high-temperature area.

A two-layer structure magnetooptical recording medium basicallycomprising the readout layer and the recording layer of the presentinvention is first described below.

With regard to a magnetic thin film comprising a single layer, it isknown that a chief magnetization direction is determined by an effectiveperpendicular magnetic anisotropy constant K⊥ that is defined by thefollowing equation 1:K⊥=Ku−2πMs²   (1)wherein Ms is the saturation magnetization of the magnetic thin film,and Ku is the perpendicular magnetic anisotropy constant. When K⊥ ispositive, the magnetic film becomes a perpendicular magnetization film.When K⊥ is negative, the magnetic film becomes an in-plane magnetizationfilm. Here, 2πMs² is energy of demagnetizing field.

For example, when the magnetic film has temperature dependency of Ms andKu as shown in FIG. 4, the magnetic film is an in-plane magnetizationfilm since the following equation 2 is established:Ku<2πMs², K⊥<0   (2)However, at the time of information reproduction, Ms of the readoutlayer decreases since the temperature increases. Thus, 2πMs² rapidlydecreases and becomes smaller than the perpendicular magnetic anisotropyconstant Ku, as shown by the following relation 3:Ku>2πMs², K⊥>0   (3)As a result, the readout layer becomes a perpendicular magnetizationfilm.

When the temperature further increases, the relation between 2πMs² andKu is again reversed after compensation temperature, and the followingrelation 4 is again obtained:Ku<2πMs², K⊥<0   (4)As a result, the readout layer again becomes an in-plane magnetizationfilm.

Namely, a state is realized in which the readout layer becomes anin-plane magnetization film in the highest-temperature andlow-temperature regions within a portion of the light spot, and thereadout layer becomes a perpendicular magnetization film in themedium-temperature region thereof, as shown in FIG. 5. Since the readoutlayer, which is a perpendicular magnetization film, is magneticallycoupled to the recording layer due to exchange coupling therebetween,the direction of magnetization in the readout layer is aligned along astable direction for the magnetization direction of information recordedin the recording layer. Thus, the information recorded in the recordinglayer is transferred into the readout layer. The transferred informationis converted into an optical signal by magneto-optic effect(magneto-optic effect (polar Kerr effect) of a laser beam reflected fromthe readout layer) of the readout layer, and detected. Namely, theinformation is detected by detecting the light reflected from thereadout layer. In this case, the magneto-optic effect (polar Kerreffect) would not occur in a portion within the light spot where thereadout layer is an in-plane magnetization film.

Thus, as shown in FIG. 5, a masked region for masking magnetizationinformation in the recording layer, and an aperture region for detectingthe information are formed within the light spot. Since the apertureregion can be formed to have an area smaller than the light spot,signals having a periodicity below the diffraction limit of light can bedetected, thereby increasing the linear density.

Since it is also possible to mask a mark on an adjacent track, thedensity of the adjacent track can be improved.

Although a case of magnetic coupling due to exchange coupling betweenthe readout layer and the recording layer is described above, it ispossible that the recording layer is magnetically coupled to the readoutlayer due to magneto-static coupling at the time of reproduction. Whenthe readout layer and the recording layer are layered directly or withan intermediate layer therebetween, Ku apparently increases due to theexchange coupling force or magnetostatic coupling force applied from theperpendicular magnetization film, and thus the temperature regionserving as a perpendicular magnetization film shifts to a lowertemperature side, compared with a case where the readout layer andrecording layer are not layered. If presetting a perpendicularmagnetization temperature region in a single layer film at a slightlyhigher value, even when the readout layer is layered with theperpendicular magnetization layer, it is possible that the readout layeris an in-plane magnetization film at room temperature and hightemperatures, and shifts into a perpendicular magnetization film only inthe medium temperature region.

Masking may be achieved at the highest-temperature point or region bydisappearance of magnetization in the readout layer. However, the signallevel in readout might be slightly reduced because Curie temperature Tcof the readout layer needs to be lower than Curie temperature Tc of therecording layer.

The following is an example of an improved magnetooptical recordingmedium of the present invention, which contains an intermediate layerbetween a readout layer and a recording layer as shown in FIG. 6, andthus basically comprises three magnetic film layers.

In this example, the intermediate layer is interposed between thereadout layer and the recording layer, and Curie temperature of theintermediate layer is higher than room temperature and lower than Curietemperatures of the readout and recording layers. Material for theintermediate layer may be a rare-earth and iron group amorphous alloy,such as TbFe, GdFe, TbFeCo or GdFeCo, or such an alloy into which anon-magnetic element such as Al, Cu and Cr are added.

When the readout layer and the recording layer are layered, the exchangecoupling force from the recording layer acts in a direction to make thespin (magnetization) direction of the readout layer perpendicular. Thus,the perpendicular magnetic anisotropy of the readout layer apparentlyincreases. Although this apparent increase in the perpendicular magneticanisotropy is omitted in the above description, the effectiveperpendicular magnetic anisotropy K⊥ will be handled below inconsideration of the increase.

Assuming that the thickness of the readout layer is h1, saturationmagnetization is Ms, perpendicular magnetic anisotropy constant is Ku,and energy of the interface magnetic domain walls between the readoutlayer and the recording layer is σ_(W), when the thickness of theinterface magnetic domain walls is neglected, an increase inperpendicular magnetic anisotropy of the readout layer due to exchangecoupling force is expressed by σ_(W)/(4h1).

Thus, the effective perpendicular magnetic anisotropy constant K⊥ is asfollows: $\begin{matrix}{{K\bot} = {{Ku} + \frac{\sigma\quad w}{4{h1}} - {2\quad\pi\quad{Ms}^{2}}}} & (5)\end{matrix}$

As shown in FIG. 6, the readout layer is subjected to the exchangecoupling force from the recording layer at room temperature (RT), butenergy of a demagnetizing field is dominant because of large Ms within alow-temperature region near room temperature. As a result, the followingrelation 6 is obtained, and the readout layer becomes an in-lanemagnetization film. $\begin{matrix}{{{{Ku} + \frac{\sigma\quad w}{4{h1}}} < {2\quad\pi\quad{Ms}^{2}}},{K\bot < 0}} & (6)\end{matrix}$

Similar to the above example, in a portion of the magnetoopticalrecording medium where the temperature increases due to projection ofthe readout laser beam, Ms of the readout layer decreases, and thus2πMs² rapidly decreases. As a result, the above relation is reversed, asshown by the following relation 7, and the readout layer becomes aperpendicular magnetization film. $\begin{matrix}{{{{Ku} + \frac{\sigma\quad w}{4{h1}}} > {2\quad\pi\quad{Ms}^{2}}},{K\bot < 0}} & (7)\end{matrix}$However, in a high-temperature region within the light spot, like atroom temperature, the readout layer is an in-plane magnetization film.

The intermediate layer functions as a mediator of exchange couplingforce from the recording layer to the readout layer, until its Curietemperature is reached, and information in the recording layer istransferred to the readout layer.

However, in the high-temperature portion within the light spot, thetemperature of the intermediate layer reaches its Curie temperature. Theintermediate layer has such a composition that Curie temperature isreached, or laser power is set so that Curie temperature is reached. Inthis portion, thus, the exchange coupling force is eliminated, and theperpendicular magnetic anisotropy constant of the readout layer rapidlydecreases in appearance. Therefore, the magnetization direction of thereadout layer becomes an in-plane direction again (refer to FIG. 6).Namely, the energy of the interface domain walls between the readoutlayer and the recording layer becomes 0, and the following relation (8)is obtained:Ku<2πMs², K⊥<0   (8)Like the two-layer structure, therefore, only the medium-temperatureregion becomes an aperture region, thereby realizing super-resolution.

In such a case where the intermediate layer is formed, which has lowCurie temperature, the readout layer can be an in-plane magnetizationfilm at room temperature and raised temperatures and be a perpendicularmagnetization film at intermediate temperatures therebetween in itslayered structure together with the intermediate and recording layers,thought the readout layer has no characteristic that the layer structurein its single layer state returns to an in-plane magnetization film atraised temperatures. Thus, the recording medium comprising theintermediate layer is advantageous in that material can be selected froma wider range.

Since the intermediate layer need not to contribute to the magneto-opticeffect, reproduction characteristic do not deteriorate even if Curietemperature is set to a low value.

Although, in the above description, it is assumed for convenience sakethat the width of the interface magnetic domain walls between thereadout layer and the recording layer can be neglected, the abovedescription applies to a case where the interface magnetic domain wallsenter the readout layer to have a thickness which cannot be neglected.However, when the interface magnetic domain walls between the readoutlayer and the recording layer occur on the side of the readout layer,magnetization of the recording layer is transferred to a portion of thereadout layer, as in the state of spin orientation schematically shownin FIGS. 7(a) and 7(b). If the interface magnetic domain walls becometoo thick, therefore, it is difficult to completely mask magnetizationinformation recorded in the recording layer. It is thus preferable tothicken the readout layer or increase the in-plane anisotropy in thelow-temperature region.

Description will now be made of a case where the above magnetoopticalrecording medium comprising three magnetic films is improved. In thiscase, the intermediate layer is interposed between the readout layer andthe recording layer, and the Curie temperature thereof is higher thanroom temperature and lower than Curie temperatures of the readout layerand recording layer. In addition to these conditions, the in-planeanisotropy of the intermediate layer at temperature near roomtemperature must be larger than that of the readout layer. In order toincrease in-plane anisotropy, for example, when rare-earth and irongroup alloy is used, rare-earth elements or iron group elements may bepredominant so that Ms of the intermediate layer at room temperature isincreased.

When such an intermediate layer is interposed between the readout layerand the recording layer, the interface magnetic domain walls can beenclosed in the intermediate layer from room temperature to the apertureregion, as shown in FIGS. 8(a) and 8(b).

Thus, the readout layer stably becomes an in-plane magnetization filmwithin the low-temperature region, and it is possible to completely maskmagnetization information recorded in the recording layer.

If the Curie temperature of the intermediate layer is lower than Curietemperature of the recording layer and higher by a degree which causesno cutting of exchange coupling between the readout layer and therecording layer in the medium-temperature region within the light spot,Ms of the intermediate layer is sufficiently small in themedium-temperature region, and the in-plane anisotropy thereof isdecreased, thereby increasing perpendicular magnetic anisotropy. At thereadout temperature, even when the intermediate layer itself has noperpendicular magnetic anisotropy, perpendicular magnetic anisotropy canbe imparted to the intermediate layer by magnetic coupling force fromthe recording layer and the readout layer which came to haveperpendicular magnetic anisotropy.

In the medium-temperature region, therefore, magnetization of therecording layer is transferred to the readout layer. In thehigh-temperature region, the temperature of the intermediate layerreaches Curie temperature, and exchange coupling force is eliminated, asdescribed above. As a result, the readout layer becomes an in-planemagnetization film.

As shown in FIG. 9, therefore, the mask region for masking magnetizationinformation recorded in the recording layer, and the aperture region fordetecting the magnetization information are formed within the lightspot. Since the aperture region can be formed to have an area smallerthan the light spot, signals with periodicity below the diffractionlimit of light can be detected. Further, as described above, the maskcan completely be operated on the front side.

Since a mark on an adjacent track can completely be masked, the densityof the adjacent track can further be improved.

In this case, the intermediate layer is preferably formed by usingmaterial such as a Gd alloy or the like which has low anisotropy andeasily forms interface magnetic domain walls, for example, GdFe, GdFeCoor the like, or the material to which a non-magnetic element such as Al,Cu or Cr is added for decreasing Curie temperature.

The thickness of the intermediate layer may be equal to or more than thethickness of the interface magnetic domain walls between the readoutlayer as an in-plane magnetization film and the recording layer as aperpendicular magnetization film. On the other hand, if the intermediatelayer is too thick, the total thickness of the magnetic layers isincreased, thereby necessitating much power for recording. Theexcessively thick intermediate layer is thus undesirable. The thicknessof the intermediate layer is preferably 20 A to 200 A, more preferably50 A to 150 A.

In regard to the physical properties of the readout layer, theintermediate layer and the recording layer, assuming that the Curietemperatures of the readout layer, the intermediate layer and therecording layer are T1, T3 and T2; the compensation temperature of thereadout layer is Tcomp1; the saturation magnetizations of the readoutlayer, the intermediate layer and the recording layer are Ms1, Ms2 andMs3, effective perpendicular magnetic anisotropy constants are K⊥1, K⊥3and K⊥2; and the energy values of perpendicular magnetic anisotropy isKu1, Ku3 and Ku2; the following equation (9) is obtained:K⊥=Kui−2πMsi² (i=1, 2, 3)   (9)At room temperature, the effective perpendicular magnetic anisotropyconstants K⊥1, K⊥3 and K⊥2 may have the following relation:K⊥3<K⊥1<<K⊥2   (10)At room temperature, examples satisfying the above relation are asfollows:Ms1<Ms3   (11)Ms2<Ms3   (12)

In addition, as described above, the Curie temperatures must satisfy therelation (13) below.RT (room temperature)<Tc3<<Tc1   (13)

FIGS. 10(a) through 10(c)1 show an example of temperature tendencies ofsaturation magnetization of the readout layer, the intermediate layerand the recording layer, which satisfy the above conditions.

In order to decrease the in-plane anisotropy of the intermediate layer,as described above, Ms may be increased, or energy of perpendicularmagnetic anisotropy Ku may be decreased or made negative (in-planeanisotropy) by adding elements such as Co and the like, which increasein-plane anisotropy.

As described above, in the information reproducing method using themagnetooptical recording medium of the present invention, since areproducable portion within the laser beam spot is located in a narrowzone between a high-temperature portion and a low-temperature portion,it is possible to reproduce information with high resolution even if theinformation is recorded in higher density. A higher C/N ratio can alsobe expected because the detecting region is placed in a center of thelaser beam spot. The reason for this is explained hereinafter.

Intensity distribution of the laser beam is of a Gaussion type and theintensity at a center thereof is highest. Thus, the closer to the centerof the spot a position, where information is reproduced, is, the betterthe C/N ratio becomes.. Generally, the center of the spot is coincidentwith the center of temperature distribution of the medium when therecording medium moves. The highest temperature point shifts toward themoving direction of the medium. Therefore, when the highest-temperaturepoint is selected as a detectable area, the detecting area will bedeviated from the center of the spot (FIG. 12).

Although the present invention is described in detail below withreference to experimental examples, the present invention is not limitedto these experimental examples within the scope of the gist of theinvention.

(First Experimental Example)

Targets Si, Tb, Gd, Fe, Co, Al and Cu were installed in a DC magnetronsputtering equipment, and a glass substrate was held on a holder.Thereafter, air was vacuum-exhausted from a chamber to establish a highvacuum level of less than 1×10⁻⁵ Pa by using a cryosorption pump.

Ar gas was introduced into the chamber while vacuum-exhausting air,until the level of 0.3 Pa or Ar gas was reached. Then, a SiN layer,which functioned as an interference dielectric film, was deposited to athickness of 700 Å on the surface of the substrate. A GdFeCo layer was(thickness: 400 Å) was deposited as a readout layer, and a TbFeCo layer(thickness: 400 Å) was deposited as a recording layer. Then, another SiNlayer (thickness: 800 Å), which functioned as a protective dielectricfilm, was deposited to form a magnetooptical recording medium of thepresent invention having the two-layer structure shown in FIG. 3(a).

When the SiN layer was formed, N₂ gas was introduced in addition to theAr gas and the deposition is performed by DC reactive sputtering. TheGdFeCo layer and the TbFeCo layer were formed by applying DC powers tothe targets of Gd, Fe, Co and Tb, respectively.

The composition of the GdFeCo layer was adjusted to that itscompensation and Curie temperatures were 240° C. and over 400° C.,respectively.

The composition of the TbFeCo layer was adjusted to that itscompensation and Curie temperatures were less than room temperature and230° C., respectively.

It was found that Kerr effect (residual Kerr rotation angle), when nomagnetic field was applied, appeared only in a temperature range from130° C. to 180° C., as shown in FIG. 13, and a perpendicularmagnetization film was established, by measuring the residual θ_(K) atthe time of magnetic field=zero as the temperature of the layered filmswas raised.

(Second Experimental Example)

A magnetooptical recording medium was fabricated, which had the samelayer structure as the above first example except that a polycarbonatesubstrate having a diameter of 130 mm and pregrooves was used.

Results of measurement of recording-reproducing characteristics of themagnetooptical recording medium were as follows. A measuring instrumentcomprised an objective lens of 0.55 N.A. and a projector for outputtinga laser beam of 780 mm wavelength. Power for recording was preset at 8mW, and linear velocity was 9 m/sec. Then, 6-15 MHz carrier signal wasrecorded in the recording layer by using a field modulation system inwhich a magnetic field of ±2000 e was applied stepwise. The dependencyof C/N ratio on the recorded mark length was measured. The reproducingpower was set to a value (2.5 to 3.5 mW) so that C/N ratio is maximized.

Table 1 shows the C/N ratios of the carrier signals recorded at 15 MHz(mark length: 30 μm), 11.25 MHz (mark length: 0.40 μm), and 9 MHz (marklength: 0.50 μm).

Then, crosstalk with an adjacent track was measured. The crosstalk wasexpressed as a difference between a reproduced signal in a land portionwhere a signal with a mark length of 1.0 μm was recorded, and thereproduced signal in an adjacent group portion. Results are shown inTable 1.

(Third Experimental Example)

A magnetooptical recording medium of the present invention comprising areadout layer, a recording layer and an intermediate layer with lowCurie temperature provided therebetween was fabricated and evaluated.

The same film forming instrument and film forming method as thoseemployed in the second experiment example were used. A SiN layer as aninterference dielectric layer was deposited to a thickness of 830 Å onthe surface of a polycarbonate substrate having a diameter of 130 mm andpregrooves. A GdFeCo layer (thickness: 400 Å) was deposited as a readoutlayer, a TbFeCoAl layer (thickness: 100 Å) was deposited as anintermediate layer, a TbFeCo layer (thickness: 300 Å) was deposited as arecording layer. Then, another SiN layer (thickness: 700 Å) wasdeposited as a protective dielectric layer to form a magnetoopticalrecording medium having the structure shown in FIG. 3(b).

When the SiN layer was formed, N₂ gas was introduced in addition to theAr gas, and the deposition was performed by DC reactive sputtering. TheGdFeCo layer and the TbFeCo layer were formed by applying DC power totargets of Gd, Fe, Co and Tb, respectively, and utilizing spontaneoussputtering. The compositions of those layers were controlled byadjusting the DC powers applied to the respective targets in sputteringfilm formation.

The composition of the GdFeCo readout layer was set so that itscompensation and Curie temperatures are 250° C. and over 310° C.,respectively. The composition of the TbFeCoAl intermediate layer was setso that its Curie temperature is 150° C. The composition of the TbFeCorecording layer was set so that its Curie temperature is 210° C.

The mark length dependency of C/N ratio, and crosstalk were measured bythe same method as in the second experimental example. Results are shownin Table 1.

(Fourth to Seventh Experimental Examples)

Magnetooptical recording media of the present invention having atwo-layer structure was fabricated by the same film forming equipment asthat used in the second and third experimental examples, and the marklength dependency of C/N ratios were measured by the same method. Themeasured physical property values, C/N ratios and crosstalk are shown inTable 1.

(Eighth and Ninth Experimental Examples)

Magnetooptical recording media of the present invention having athree-layer structure comprising an intermediate layer with low Curietemperature were fabricated by the same film forming instrument as thatused in the second to seventh experimental examples. The mark lengthdependencies of C/N ratios were measured by the same method as in thesecond to seventh experimental examples. The measured physical propertyvalues, C/N ratios and crosstalk of the respective layers are shown inTable 1.

(Tenth Experimental Example)

A magnetooptical recording medium of the present invention having athree-layer structure comprising an intermediate layer with low Curietemperature and in-plane anisotropy which was larger than that of areadout layer in a low-temperature region within an light beam spot, wasfabricated by the same method as that employed in the second to ninthexperimental examples.

A SiN dielectric layer of 900 Å, a GdFeCo readout layer of 400 Å, a GdFeintermediate layer of 100 Å, a TbFeCo recording layer of 300 Å and a SiNprotective layer were successively formed on a glass substrate to form asample having the structure shown in FIG. 3(b). When each of the SiNlayers was formed, a N₂ gas was introduced in addition to a Ar gas, andthe deposition was performed by DC reactive sputtering. The mixing ratiobetween the Ar gas and N₂ gas was adjusted so that the refractive indexis 2.1.

The composition of the GdFeCo readout layer was set so that the layer isRE-rich and has saturation magnetization Ms of 160 emu/cc at roomtemperature, and its compensation temperature and Curie temperature are205° and over 300° C., respectively.

The composition of the TbFe intermediate layer was set so that the layeris RE-rich and has a saturation magnetization Ms of 520 emu/cc at roomtemperature, and its Curie temperature is 150° C.

The composition of the TbFeCo recording layer was set so that the layeris TM-rich and has a saturation magnetization of 200 emu/cc, and itsCurie temperature is 220° C.

The dependency of the Kerr rotation angle (θ_(K)) on an externalmagnetic field was measured by applying a semiconductor laser beam of830 nm to a sample having layers, which were formed on a glass substrateby the above method, from the side of the glass substrate. Themeasurement was performed by heating the sample from room temperature toabout 200° C. FIG. 13 shows the temperature dependency of the Kerrrotation angle (residual Kerr rotation angle: θ_(K) ^(R)) at the time ofexternal magnetic field=0. It is found from FIG. 13 that the residualKerr rotation angle θ_(K) ^(R) is substantially zero from roomtemperature to about 140° C., then rapidly increases from about 140° C.and becomes zero at about 200° C.

(Eleventh Experimental Example)

A magnetooptical recording film having the same layer structure andlayer compositions as those in the tenth experimental example was formedon a polycarbonate substrate with pregrooves to form a magnetoopticalrecording medium of the present invention.

The dependency of C/N ratio on the recorded mark length and crosstalkwere measured by the same method as in the second to ninth experimentalexamples. Results are shown in Table 1.

(Twelfth Experimental Example)

A SiN dielectric layer of 900 Å, a GdFeCo readout layer of 400 Å, a GdFeintermediate layer of 120 Å, a TbFeCo recording layer of 300 Å, and aSiN protective layer of 700 Å were successively formed on apolycarbonate substrate by the same instrument and method as thoseemployed in the first experimental example to obtain a sample having thestructure shown in FIG. 3(b).

The composition of the GdFeCo readout layer was set so that the layer isRE-rich and has a saturation magnetization Ms of 180 emu/cc at roomtemperature, and its compensation temperature and Curie temperature are220° C. and over 300° C., respectively.

The composition of the GdFe intermediate layer was set so that the layeris RE-rich and has a saturation magnetization Ms of 680 emu/cc at roomtemperature, and its Curie temperature is 180° C.

The composition of the TbFeCo recording layer was set to that the layeris TM-rich and has a saturation magnetization Ms of 200 emu/cc at roomtemperature, and its Curie temperature is 220° C.

TABLE 1 Readout layer Intermediate layer Thick Ms T_(cx×n) Tc Thick MsTc Composition Å e/cc ° C. ° C. Composition Å e/cc ° C. Example 1,2Gd₃₂(Fe₅₅Co₄₅)₆₉ 400 — 240 400< — — — — Example 3 Gd₃₀(Fe₆₀Co₄₀)₇₀ 400 —250 310< (Tb₂₄(Fe₉₅Co₅)₇₆)₉₅Al₅ 100 100 150 Example 4 Gd₂₈(Fe₆₅Co₃₅)₇₂350 — 205 300< — — — — Example 5 (Gd₇₃Tb₂₇)₇₀Co₃₀ 300 — 205 300< — — — —Example 6 Gd₂₈(Fe₆₀Co₄₅)₇₂ 400 — 205 300< — — — — Example 7(Nd₁₀Gd₉₀)₃₀(Fe₆₀Co₄₀)₇₀ 370 — — 300< — — — — Example 8 Gd₂₉(Fe₅₀Co₅₀)₇₁400 — — 300< (Tb₂₃(Fe₉₄Co₆)₇₇)₉₄Cu₆ 50  80 170 Example 9Gd₂₈(Fe₇₀Co₃₀)₇₂ 360 260 — 300< Gd₄₀Fe₆₀ 80 460 188 Example 10,11Gd₂₅(Fe₆₀Co₄₀)₇₂ 400 180 205 300< Gd₄₅Fe₅₅ 100  520 150 Example 12Gd₂₉(Fe₆₀Co₄₀)₇₁ 400 200 220 300< Gd₄₅(Fe₉₀Co₁₀)₅₀Al₅ 120  680 180Example 13 Gd₂₇(Fe₆₈Co₃₂)₇₃ 400 150 188 300< Gd₄₅(Fe₉₈Co₂)₅₅ 80 520 170Example 14 Gd₂₇(Fe₆₅Co₃₅)₇₃ 400 160 188 300< Gd₄₀(Fe₉₄Co₆)₅₀ 90 470 165Co. Ex. 1,2 Gd₂₇(Fe₆₅Co₃₅)₇₃ 400 130 280 300< — — — — Co. Ex. 3Gd₃₇(Fe₆₀Co₄₀)₆₈ 400 270 280 300< — — — — Recording layer C/N (dB) ThickMs Tc 0.40 μ Cross-talk Composition Å e/cc ° C. 0.30 μ dB 0.50 μ dBExample 1,2 Tb₂₀(Fe₈₀Co₂₀)₅₀ 400 −200 230 30 33 44 −30 Example 3Tb₂₀(Fe₈₀Co₂₀)₅₀ 300 −200 210 36 41 47 −35 Example 4 Tb₂₀(Fe₈₀Co₂₀)₅₀370 −200 220 30 34 45 −31 Example 5 Tb₂₀(Fe₈₀Co₂₀)₅₀ 400 −200 220 30 3344 −30 Example 6 Dy₂₀(Fe₈₀Co₂₀)₅₀ 380 −200 220 31 32 44 −29 Example 7Tb₂₀(Fe₈₀Co₂₀)₅₀ 400 −200 220 30 31 46 −28 Example 8 Tb₂₀(Fe₈₀Co₂₀)₅₀450 −200 220 35 41 46 −36 Example 9 Tb₂₀(Fe₈₀Co₂₀)₅₀ 300 −200 220 39 4447 −35 Example 10,11 Tb₂₀(Fe₈₀Co₂₀)₅₀ 300 −200 220 41 45 48 −40 Example12 Tb₂₀(Fe₈₀Co₂₀)₅₀ 300 −200 220 39 44 48 −41 Example 13Tb₂₀(Fe₈₀Co₂₀)₅₀ 300 −200 220 40 45 48 −40 Example 14 Tb₂₀(Fe₈₀Co₂₀)₅₀300 −200 220 40 44 48 −41 Co. Ex. 1,2 Tb₂₀(Fe₈₀Co₂₀)₅₀ 300 −200 220 2026 46 −20 Co. Ex. 3 Tb₂₀(Fe₈₀Co₂₀)₅₀ 300 −200 220 26 29 47 −21 e/cc =emu/cc

Then, the dependency of C/N ratio on the recorded mark length andcrosstalk were measured by the same method as in the second to ninthexperimental examples. Results are shown in Table 1.

(Thirteenth Experimental Example)

A SiN dielectric layer of 900 Å, a GdFeCo readout layer of 400 Å, a GdFeintermediate layer of 80 Å, a TbFeCo recording layer of 300 Å, and a SiNprotective layer of 700 Å were successively formed on a polycarbonatesubstrate by the same instrument and method as those employed in thefirst experimental example to obtain a sample having the structure shownin FIG. 3(b).

The composition of the GdFeCo readout layer was set so that the layer isRE-rich and has a saturation magnetization Ms of 150 emu/cc at roomtemperature, and its compensation temperature and Curie temperature are188° C. and over 300° C., respectively.

The composition of the GdFe intermediate layer was set so that the layeris RE-rich and has a saturation magnetization Ms of 520 emu/cc at roomtemperature, and its Curie temperature is 170° C.

The composition of the TbFeCo recording layer was set so that the layeris TM-rich and has a saturation magnetization Ms of 200 emu/cc at roomtemperature, and its Curie temperature is 220° C.

Then, the dependency of C/N ratio on the recorded mark length andcrosstalk were measured by the same method as in the second to ninthexperimental examples. Results are shown in Table 1.

(Fourteenth Experimental Example)

A SiN dielectric layer of 900 Å, a GdFeCo readout layer of 400 Å, a GdFeintermediate layer of 90 Å, a TbFeCo recording layer of 300 Å, and a SiNprotective layer of 700 Å were successively formed on a polycarbonatesubstrate by the same instrument and method as those employed in thefirst experimental example to obtain a sample having the structure shownin FIG. 3(b).

The composition of the GdFeCo readout layer was set so that the layer isRE-rich and has a saturation magnetization Ms of 160 emu/cc at roomtemperature, and its compensation temperature and Curie temperature are188° C. and over 300° C., respectively.

The composition of the GdFe intermediate layer was set so that the layeris RE-rich and has a saturation magnetization Ms of 470 of emu/cc atroom temperature, and its Curie temperature is 165° C.

The composition of the TbFeCo recording layer was set so that the layeris TM-rich and has a saturation magnetization Ms of 200 emu/cc at roomtemperature, and its Curie temperature is 220° C.

Then, the dependency of C/N ratio on the recorded mark length andcrosstalk were measured by the same method as in the second to ninthexperimental examples. Results are shown in Table 1.

(First Comparative Experimental Example)

A SiN dielectric layer of 900 Å, a GdFeCo readout layer of 400 Å, aTbFeCo recording layer of 300 Å, and a SiN protective layer of 700 Åwere successively formed on a polycarbonate substrate by the sameinstrument and method as those employed in the first experimentalexample to obtain a sample having the structure shown in FIG. 3(a).

The composition of the GdFeCo readout layer was set so that the layer isRE-rich and has a saturation magnetization Ms of 130 emu/cc at roomtemperature, and its compensation temperature and Curie temperature are280° C. and about 300° C., respectively.

The composition of the TbFeCo recording layer was set so that the layeris TM-rich and has a saturation magnetization Ms of 200 emu/cc at roomtemperature, and its Curie temperature is 220° C.

This sample had the temperature dependency of residual θ_(K) as shown inFIG. 15, and did not become again an in-plane magnetization film at hightemperatures. As in this comparative example, in a recording mediumhaving a two-layer structure comprising a readout layer and a recordinglayer in which the compensation temperature and Curie temperature areclose to each other, the readout layer cannot be made an in-planemagnetization film at high temperatures.

(Second Comparative Experimental Example)

Layers were formed on a polycarbonate substrate to form a magnetoopticalrecording medium by the same method as that employed in the secondexperimental example. Then, the dependency of C/N ratio on the recordedmark length and crosstalk were measured by the same method as in thesecond to ninth experimental examples. Results are shown in Table 1.

(Third Comparative Experimental Example)

A SiN dielectric layer of 900 Å, a GdFeCo readout layer of 400 Å, aTbFeCo recording layer of 300 Å, and a SiN protective layer of 700 Åwere successively formed on a polycarbonate substrate by the sameinstrument and method as those employed in the first experimentalexample to obtain a sample having the structure shown in FIG. 3(a).

The composition of the GdFeCo readout layer was set so that the layer isRE-rich and has a saturation magnetization Ms of 180 emu/cc at roomtemperature, and its compensation temperature and Curie temperature are290° C. and about 300° C., respectively.

The composition of the TbFeCo recording layer was set so that the layeris TM-rich and has a saturation magnetization Ms of 200 emu/cc at roomtemperature, and its Curie temperature is 220° C.

Then, the dependency of C/N ratio on the recorded mark length andcrosstalk were measured by the same method as in the second to ninthexperimental examples. Results are shown in Table 1.

Comparison of the experimental examples 2 to 14 and comparativeexperimental examples 2 and 3 reveals that the present invention cansignificantly improve the C/N ratio and crosstalk with a short marklength.

Use of the magnetooptical recording medium and reproducing method of thepresent invention enable reproduction of a magnetic domain smaller thanthe diameter of a beam spot by using a simple instrument (conventionalinstrument) which requires no initialization magnet, and achievement ofhigh-density recording in which the linear recording density and trackdensity are further improved, thereby improving the C/N ratio.

1. A magnetooptical recording medium adapted to be heated from a roomtemperature range to a medium temperature range above the roomtemperature range and to a high temperature range above the mediumtemperature range, said medium comprising: a first magnetic layer, whichhas an in-plane magnetization at the room temperature range, and whichchanges to a perpendicular magnetization at the medium temperaturerange; a second magnetic layer having a perpendicular magnetization; anda third magnetic layer, wherein the third magnetic layer is interposedbetween said first and second magnetic layers, and has a Curietemperature lower than those of said first and second magnetic layers,and has an in-plane magnetization at the room temperature range andchanges to a perpendicular magnetization at the medium temperaturerange.
 2. A method of reproducing, with a laser beam, informationrecorded on a magnetooptical recording medium comprising a firstmagnetic layer, a second magnetic layer having a perpendicularmagnetization, and an intermediate layer therebetween having a Curietemperature higher than a room temperature range, lower than the Curietemperature of the first and second magnetic layers, and in a hightemperature range, the first magnetic layer having an in-planemagnetization at the room temperature range, changing to a perpendicularmagnetization at a medium temperature range higher than the roomtemperature range and changing back to an in-plane magnetization at orabove the Curie temperature of the intermediate layer in the hightemperature range higher than the medium temperature range, said methodcomprising the steps projecting a laser beam onto the magnetoopticalrecording medium from a side of the first magnetic layer; heating thefirst magnetic layer with the laser beam so that the first magneticlayer has a portion in the room temperature range having in-planemagnetization and a portion in the medium temperature range having aperpendicular magnetization; heating a portion of the intermediate layerat least to its Curie temperature so that a corresponding portion of thefirst magnetic layer in the high temperature range changes to anin-plane magnetization; transferring information recorded in the secondmagnetic layer to the first magnetic layer by exchange coupling throughthe intermediate layer perpendicular magnetization of the first magneticlayer and magnetization of the second magnetic layer; and reproducingthe recorded information based on the magneto-optic effect of the lightreflected from the magnetooptical recording medium.
 3. Themagnetooptical recording medium of claim 1, wherein the first magneticlayer and the third magnetic layer are magnetically coupled by exchangecoupling.